Contamination free source for shallow low energy junction implants

ABSTRACT

A method for forming a P-type region in a semiconducting crystalline substrate by ion implantation is disclosed, wherein the implant specie is an ionic molecule that contains titanium and boron.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of integrated circuitmanufacturing and, more particularly, to ion implantation.

2. Description of the Related Art

Ion implantation is a well known technique for forming junctions insemiconductors. During ion implantation, a source material is used toproduce ions which are ultimately implanted into a silicon wafer. Wherethe ions become embedded in the silicon, they change the silicon'selectrical properties. If an N-type source material is used (e.g., onecontaining phosphorous), the resulting silicon will become N-type, orrich with electrons. If a P-type source material is used (e.g., onecontaining boron), the resulting silicon will become P-type, or richwith "holes."

P-type junctions have traditionally been manufactured by the use of aboron-based source gas, usually boron trifluoride, or BF₃. Inside of anion implantation machine, the source gas is ionized, creating, amongstother species, BF₂ ⁺ ions. In traditional Ultra Large Scale Integration(ULSI) applications requiring P-type junctions, the BF₂ ⁺ species isused as the implant species in the formation of boron-doped, P-typejunctions. The BF₂ ⁺ species can be extracted by adjusting the fieldstrength of extractor magnet inside of the ion implanter so as to choosean ion species of the desired charge-to-mass ratio.

However, using the BF₂ ⁺ ion as the implant species has its drawbacks.Most importantly, molybdenum is usually present in ion implantationmachine components and acts as a source of contamination. An ionizedmolybdenum ion, Mo⁺⁺, has approximately the same charge-to-mass ratio asthe BF₂ ⁺ species. Therefore, if Mo⁺⁺ ions are present in the ionplasma, the magnet will be unable to extract the desired BF₂ ⁺ specieswithout also extracting the Mo⁺⁺ species. As a result, any ionizedmolybdenum will be implanted into the silicon along with the BF₂ ⁺species. Unfortunately, the presence of molybdenum degrades theperformance of the P-type junctions formed, because the presence ofmolybdenum in the junction will increase the junction's reverse biasleakage. In DRAM technologies, for instance, increased reverse biasleakage degrades refresh characteristics. Junction degradation issuesbecome particularly acute in the ULSI era, which demands shallowjunctions of high purity.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a method for forming a P-type region in a substrate. The methodincludes the steps of implanting a species of ionic molecule containingtitanium and boron into the substrate, and annealing the substrate tocause the boron to dissociate with the titanium.

In accordance with another aspect of the present invention, there isprovided a method for forming a titanium silicide layer. The methodincludes the steps of implanting a species of ionic molecule includingtitanium and boron into a silicon surface, and annealing the siliconsurface to cause the titanium to react with the silicon.

In accordance with a further aspect of the present invention, there isprovided a method for simultaneously forming a P-type region and atitanium silicide layer in silicon. The method includes the steps ofimplanting a species of ionic molecule containing titanium and boroninto the silicon, and annealing the silicon to cause the boron todissociate from the titanium and to cause the titanium to react with thesilicon.

In accordance with yet another aspect of the present invention, there isprovided a method for forming a silicided contact. The method includesthe steps of etching a contact to expose an underlying silicon surface,implanting a species of ionic molecule containing titanium and boroninto the silicon surface, and annealing the silicon surface to cause thetitanium to react with the silicon surface.

In accordance with still another aspect of the present invention, thereis provided a method for forming a titanium silicided contact to aP-type region. The method includes the steps of etching a contact toexpose an underlying silicon surface, implanting a species of ionicmolecule containing titanium and boron into the silicon surface, andannealing the silicon surface to cause the boron to dissociate from thetitanium and to cause the titanium to react with the silicon surface.

In accordance with a still further aspect of the present invention,there is provided a P-type implant. The implant includes a substratehaving an annealed implant of a species of molecule containing titaniumand boron.

In accordance with a yet further aspect of the present invention, thereis provided a titanium silicide layer. The titanium suicide layerincludes a silicon substrate having an annealed implant of a species ofmolecule containing titanium and boron.

In accordance with another aspect of the present invention, there isprovided an integrated circuit that includes a substrate and a P-typeregion in the substrate having an annealed implant of a species ofmolecule containing titanium and boron.

In accordance with a further aspect of the present invention, there isprovided an integrated circuit that includes a silicon surface and atitanium silicide layer at the silicon surface having an annealedimplant of a species of molecule containing titanium and boron.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain advantages of the invention will become apparent upon readingthe following detailed description of specific embodiments and uponreference to the drawings in which:

FIG. 1 is a cross sectional representation of an ion implanter showingits component parts.

FIG. 2 is a cross sectional view of a silicon wafer showing the statusof the wafer after boron dissociation and silicidation afterpost-implant annealing.

FIG. 3 is a cross sectional view of a contact that has been silicided bythe use of the disclosed process.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In one embodiment, a combination of titanium tetrachloride (TiCl₄) anddiborane (B₂ H₆) source materials are used to create a TiB_(x) ⁺ speciessuitable for forming a shallow and pure P-type junction. To facilitatethe description of the disclosed embodiment, FIG. 1 shows arepresentation of an ion implanter 1. Source materials enter the ionimplanter 1 through port 2 that is coupled to a source chamber 3. Inconventional ion implantation procedures, the source materials areusually gases. The gases are ionized into a gas plasma in the sourcechamber 3. The positively ionized ion species are then extracted by anegatively biased extractor 4 and sent to a charge-to-mass analyzer 5.The charge-to-mass analyzer 5 provides a magnetic field which isdesigned to select an ion species of a certain charge-to-mass ratio andsend that species into the accelerator 6. Those ion species 7 which donot have the proper charge-to-mass ratio are deflected away from theaccelerator 6. Those chosen ion species which reach the accelerator 6are accelerated for eventual impact into the silicon wafer 8.Electrostatic deflectors 9 allow the ion beam to be rastered, orscanned, over the surface of the silicon wafer 9 for uniform ionimplantation over the surface of the silicon wafer 8. Various pumps 10are used to control the pressures inside of the different regions of theion implanter 1. Example of standard ion implanters capable ofperforming this method are the Varian™ E1000 and E500 models.

In the illustrated embodiment, TiCl₄ and B₂ H₆ are used as the sourcematerials, although other Ti and boron sources may be substituted forTiCl₄ and B₂ H₆. While B₂ H₆ is a gas, TiCl₄ is a liquid. To produce asuitable amount of TiCl₄ gas to partake in ionization, the TiCl₄ liquidis heated to 40 degrees Centigrade. Because TiCl₄ has a relatively highvapor pressure, treatment of the TiCl₄ under these conditions willproduce a sufficient amount of TiCl₄ gas to participate in ionization.Typically, the TiCl₄ source material is converted into vapor outside ofthe ion implanter 1 before being administered into the source chamber 3.

After a sufficient amount of TiCl₄ and B₂ H₆ gases is introduced intothe source chamber 3, both the TiCl₄ and B₂ H₆ reactant species areionized. The ionization of the reactant species will give rise toseveral different ionic species. After the reactant species are ionized,the ions react and form, among other species, some form of TiB_(x) ion.Examples of the TiBx ion species are TiB⁺ and TiB₂ ⁺, but similar ionswith a double-positive charge may also exist (e.g., TiB₂ ⁺⁺) dependingon the RF power provided to the plasma. The use of the TiB₂ ⁺ species inthe disclosed process is advantageous because it should exist in higherconcentrations than, for example TiB₂ ⁺⁺, and is also advantageousbecause of its mass, as will be explained later. The TiB₂ ⁺ species isthen extracted and accelerated through the ion implanter 1 and isultimately implanted into the silicon wafer 8.

The use of these source materials exhibits distinct advantages. Becausea BF₂ ⁺ species is not used, molybdenum contamination is eradicated.Because the TiB₂ ⁺ species has a different charge to mass ratio thandoes Mo⁺⁺, the extraction of the TiB₂ ⁺ species will not simultaneouslyextract any Mo⁺⁺ species that happens to be present. Also, because theTiB₂ ⁺ species has a relatively high molecular weight when compared withBF₂ ⁺, shallower implants are created for a given accelerating voltage,such as that produced by accelerator 6. Moreover, because the TiB₂ ⁺species is heavier than BF₂ ⁺, the TiB₂ ⁺ species will create moredamage to the silicon crystalline surfaces upon impact. This disruptionof the crystalline lattice reduces or prevents ion channeling commonwith ion implantation procedures and helps to keep the junction shallow.

After implantation, an anneal process is performed. Typical, but notexclusive parameters for this rapid thermal anneal process aredisclosed: time=5 to 60 seconds; temperature=500 to 900 degreesCentigrade; ambient=argon or other inert gases. Nitrogen based gases canalso be used as the ambient, and these will form a thin barrier oftitanium nitride over exposed silicon surfaces, which may be beneficialdepending on the process at issue. A furnace anneal for 1 to 120 minutesat 500 to 900 degrees Centigrade may also be used. Furthermore, acombination of a plasma anneal, rapid thermal anneal, and a furnaceanneal may be used.

The anneal serves a number of purposes, as shown in the wafercross-section illustrated in FIG. 2. First, the anneal heals the damageto the silicon crystalline lattice 22 that occurs when the TiB₂ ⁺ 23enters the crystal. Second, the anneal allows the boron to dissociatefrom the titanium and diffuse through the silicon crystalline lattice 22to form the desired P-type region 20. Third, the titanium, which is arelatively large molecule, will not be as prone to diffusing in thesilicon and instead will stay at the original silicon surface. The hightemperature of the anneal will allow the titanium to react with thesilicon in the crystal, thus forming a titanium silicide alloy(TiSi_(x)) 21 at the surface of the silicon.

The simultaneous formation of TiSi_(x) upon anneal after the use of thedisclosed process has a number of advantages. First, P-type junctionscan be created which do not require a separate silicidation process.Because a silicide is already formed by virtue of implantation of TiB₂ ⁺and anneal, the resulting P-type junctions are not only shallow andpure, but are of low resistance as well. Low resistance junctions arebeneficial to improving chip speed, as one skilled in the art is aware.

Second, implantation of TiB₂ ⁺ can also be used to form low resistancecontacts as well. In ULSI technologies, it is becoming more difficult toform contacts of sufficiently low resistance due to the high aspectratio of the contacts. Referring to FIG. 3, a contact hole 31 has beenanisotropically etched through an oxide 32 to expose either anunderlying transistor gate or junction (a junction 37 is shown in FIG.3). The aspect ratio is defined as the height of the contact 33 dividedby the width of the contact 34. As contact widths grow smaller in ULSItechnologies, the aspect ratio grows higher. High aspect ratios make itdifficult to sputter metals, such as titanium, into the bottom 36 of thecontacts. Because sputtered titanium typically provides poor coverageinside of the contacts, the resulting contacts will have undesirablyhigh resistances, which can degrade chip speed. However, by implantingTiB₂ ⁺ into the contact holes 31, a uniform titanium silicide layer 36can be produced by the post-implantation anneal, thus improving contactresistance while obviating the need for a separate silicidation processstep.

Silicidation of contacts by implantation of TiB₂ ⁺, as shown in FIG. 3,can be performed in a number of ways. In one embodiment, TiB₂ ⁺implantation can take place after a junction 37 has already been formedby traditional methods. Adopting this approach, the junctions 37 arefirst formed. Then, an oxide 32 is formed and etched to form contactholes 31. The TiB₂ ⁺ implantation and anneal are performed to form atitanium silicide 36.

Because boron will disassociate from the titanium silicide 36, thepresence of the extra boron dopants should be taken into account wheninitially forming the junctions 37. For example, if the pre-existingjunction 37 was a P-type junction, the presence of the dissociated boronwould cause the junction 37 to become more heavily doped. This maydictate that the P-type junctions should initially be formed with alower doping level than that which is ultimately desired, thus takinginto account the fact that extra boron will be introduced into thejunction by implantation and anneal. If the pre-existing junction 36 wasa N-type junction, the presence of dissociated boron would counter-dopethe N-type junction. This may dictate that the N-type junction shouldinitially be formed with a higher doping level than that which isultimately desired, thus taking into account the fact that boron will beintroduced into the junction by implantation and anneal. N-typejunctions can be masked off such that only the P-type junctions areimplanted, if counter-doping of the N-type junctions is undesirable.

In another embodiment, TiB₂ ⁺ implantation may be used to formed theP-type junctions, as well to provide the titanium used to form atitanium silicide. Adopting this approach, an oxide 32 is etched to formcontact holes 31, and then TiB₂ ⁺ is implanted and annealed, forming, 20simultaneously, titanium silicide 36 and a P-type junction 37. Onebenefit of this embodiment is that the P-type junctions 37 thus formedare self-aligned. As mentioned above, N-type junctions are formed byother methods, and may be masked off during the implantation process(unless N-type junction contact silicidation is desired).

Which embodiment should be chosen is subject to a number a factors,including manufacturing cost and junction engineering considerations. Itshould also be noted that through the use of either of the disclosedembodiments, the contact etch will also, in some manufacturingprocesses, simultaneously expose the transistor gates. Because mostmodem day transistor gates are composed of polysilicon, TiB₂ ⁺implantation into the gates will form a silicide, which should bebeneficial by forming low resistance gate contacts. Extra-doping orcounter-doping of the polysilicon is not expected to have the samerepercussions as for junctions, since the degenerative nature of thepolysilicon is less susceptible to such effects.

Although the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. A method for forming a P-type region in asubstrate, comprising the steps of:(a) implanting a species of ionicmolecule containing titanium and boron into the subtrate; and (b)annealing the substrate to cause the boron to dissociate from thetitanium, thereby forming said P-type region.
 2. The method of claim 1,wherein the species of ionic molecule is TiB₂ ⁺.
 3. The method of claim1, wherein the species of ionic molecule is TiB⁺.
 4. The method of claim1, further comprising the steps of:forming the species of ionic moleculefrom TiCl₄ and B₂ H₆.
 5. The method of claim 4, wherein the step offorming comprises the step of partially converting the TiCl₄ to agaseous state through vapor pressure dissociation.
 6. A method forforming a titanium silicide layer, comprising the steps of:(a)implanting a species of ionic molecule containing titanium and boroninto a silicon surface; and (b) annealing the silicon surface to causethe titanium to react with the silicon, thereby forming said titaniumsilicide layer.
 7. The method of claim 6, wherein the species of ionicmolecule is TiB₂ ⁺.
 8. The method of claim 6, wherein the species ofionic molecule is TiB⁺.
 9. The method of claim 6, further comprising thestep of:forming the species of ionic molecule from TiCl₄ and B₂ H₆. 10.The method of claim 9, wherein the step of forming comprises the step ofpartially converting the TiCl₄ to a gaseous state through vapor pressuredissociation.
 11. A method for simultaneously forming a P-type regionand a titanium silicide layer in silicon, comprising the steps of:(a)implanting a species of ionic molecule containing titanium and boroninto the silicon; and (b) annealing the silicon to cause the boron todissociate from the titanium and to cause the titanium to react with thesilicon, thereby forming said P-type region and said titanium silicidelayer.
 12. The method of claim 11, wherein the species of ionic moleculeis TiB₂ ⁺.
 13. The method of claim 11, wherein the species of ionicmolecule is TiB⁺.
 14. The method of claim 11, further comprising thestep of:forming the species of ionic molecule from TiCl₄ and B₂ H₆. 15.The method of claim 14, wherein the step of forming comprises the stepof partially converting the TiCl₄ to a gaseous state through vaporpressure dissociation.
 16. A method for forming a silicided contact,comprising the steps of:(a) etching a contact to expose an underlyingsilicon surface; (b) implanting a species of ionic molecule containingtitanium and boron into the silicon surface; and (c) annealing thesilicon surface to cause the titanium to react with the silicon surfacethereby forming said silicided contact.
 17. The method of claim 16,wherein the species of ionic molecule is TiB₂ ⁺.
 18. The method of claim16, wherein the species of ionic molecule is TiB⁺.
 19. The method ofclaim 16, further comprising the step of:forming the species of ionicmolecule from TiCl₄ and B₂ H₆.
 20. The method of claim 19, wherein thestep of forming comprises the step of partially converting the TiCl₄ toa gaseous state through vapor pressure dissociation.
 21. A method forforming a titanium silicided contact to a P-type region, comprising thesteps of:(a) etching a contact to expose an underlying silicon surface;(b) implanting a species of ionic molecule containing titanium and boroninto the silicon surface; and (c) annealing the silicon surface to causethe boron to dissociate from the titanium and to cause the titianium toreact with the silicon surface thereby forming said forming titaniumsilicided contact to said P-type region.
 22. The method of claim 21,wherein the species of ionic molecule is TiB₂ ⁺.
 23. The method of claim21, wherein the species of ionic molecule is TiB⁺.
 24. The method ofclaim 21, further comprising the step of:forming the species of ionicmolecule from TiCl₄ and B₂ H₆.
 25. The method of claim 24, wherein thestep of forming comprises the step of partially converting the TiCl₄ toa gaseous state through vapor pressure dissociation.